Seismic exploration of the earth for oil, gas, and other valuable minerals is commonly performed. In these techniques, energy is imparted to the earth at a first location, termed a "shot point", travels into the earth, is reflected at interfaces between rock layers in the earth, returns to the surface and is detected by a number of detectors spread along a line extending from the shot point. Where land based exploration is being carried out, typically the energy is imparted to the earth by detonating a charge of dynamite, dropping a heavy weight or vibrating a massive object at the surface of the earth, and the detectors are normally termed "geophones". Where water-based exploration is being carried out, energy is imparted to the earth by releasing a quantity of compressed air into the water at regular intervals, and the detectors are termed "hydrophones". In either case, the signals output by the detectors are recorded individually as a function of time for subsequent processing.
The fact that the seismic energy must travel deeply into the earth and be reflected at interfaces between the rock layers and travel upwardly to be detected by the detectors means inevitably that the detected signal strength is relatively weak and, correspondingly, that the signal-to-noise ratio of the signals is normally rather low. The art has developed numerous techniques for removing noise from seismic records. Many of these techniques rely on the fact that much of the noise is uncorrelated, i.e., essentially random. Therefore, if similar records can be selected and summed together, normally the random noise will cancel out, whereas the signal will be strengthened. This process is usually referred to as stacking. The art also recognizes that often the seismic records are corrupted by strong steady state signals, e.g., 60 Hz energy from power lines, generators and the like, and the art shows methods of removing such consistent interference from the seismic records. For example, in the 60 Hz case, typically a notch filter can be applied to the data in generally known fashion to notch out the interfering frequency. However, a notch filter has some minimal bandwidth in which energy is removed from the signal so that some of the data is inevitably lost.
U.S. Pat. No. 4,853,903 to one of the inventors and several other persons (hereinafter "the '903 patent"), which patent is not prior art with respect to the present application, but is incorporated herein by reference, shows a particularly preferred method of designing and applying such a notch filter to seismic data. As indicated above, the notch filter described in that patent removes energy from the seismic records over some finite bandwidth, not merely the precise frequency of interest, and therefore some data is inevitably lost in its use.